1. Technical Field
This invention relates to a hologram scanner which is employed to read bar codes in a POS or UPC system or the like.
2. Description of the Prior Art
A conventional hologram scanner of this type is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a laser oscillator; 2, a light beam; 3, a lens; 4 a mirror; 5, a mirror having a hole 5a through which the light beam 2 advances; 6, hologram lenses for deflecting the light beam 2; 7, a hologram disk containing the hologram lenses 6, the centers of which are displaced in correspondence to the pitch of a scanning line, and the number of which is equal to the number of scanning lines; and 8 and 9, first and second scanning beams which are diffracted to spatially separated positions, respectively. Further in FIG. 1, reference numerals 10 and 11 designate mirrors for reflecting the scanning beams 8 and 9, respectively; 12, a reading window; 13 and 14, scanning lines provided by the scanning beams 8 and 9, respectively; 15, a light beam scattered by a bar code on a label which is placed on the reading window; 16, a photo-detector; and 17, a motor for rotating the hologram disk 7.
In operation, the light beam 2 from the laser oscillator 1 is applied through the lens 3, the mirror 4 and the hole 5a of the mirror 5 to the hologram lenses 6. As the hologram disk 7 is turned by the motor 17, the light beam 2 is deflected by the hologram lenses on the hologram disk 7, thus providing successive scanning beams. Typical of the scanning beams thus provided are the first and second scanning beams 8 and 9 mentioned above. The direction of deflection of these scanning beams is in parallel with a tangential line at the point A (which is the incident point of the light beam) of the hologram disk 7. As the centers of the hologram lenses 6 are displaced as described above, the scanning beams 8 and 9 are shifted radially of the hologram disk 7. Accordingly, parallel scanning lines, the number of which are equal to the number of hologram lenses 6 on the hologram disk 7, are generated. By increasing the amount of displacement of the centers of the hologram lenses 6, all of the scanning lines may be provided at spatially separated positions and divided into two groups. This is to facilitate the setting of the mirrors 10 and 11 to provide two scanning line directions on the reading window 12.
The scanning beam groups, which are represented by the first and second scanning beams 8 and 9, are reflected by the first and second mirrors 10 and 11 which are set at predetermined angles and at predetermined positions, to thus provide scanning lines 13 and 14, extending in two directions, over the reading window 12.
When a label is scanned according to the above-described scanning pattern, the light beam 15 scattered by the bar code PG,4 on the label advances along the optical path of the light beam 2, thus being collected on the photo-detector 16. As a result, the scattered light beam 15 is converted into an electrical signal by the photo-detector 16. Thus, the bar code has been optically read.
FIG. 1 shows two scanning beams 8 and 9 extending from one hologram lens 6. However, it should be noted that, in practice, only one scanning line is obtained from one hologram lens. That is, for convenience in describing the prior art, FIG. 1 shows another hologram lens which is also brought into the optical path of the light beam 2 by rotation of the hologram disk 7.
In order to make it possible to read a bar code in all directions with high accuracy with such a hologram scanner, it is necessary to produce a scanning pattern having a number of scanning lines in many directions. In the case of reading a bar code label of small size, or a label of small height, or in the case of reading a non-moving label, the conventional hologram scanner does not have a sufficient number of scanning line directions. Thus, it is necessary for the conventional hologram scanner to increase its number of scanning line directions. When it is so required to modify the conventional hologram scanner, it is necessary to provide reflecting mirrors, the number of which is at least equal to the number of required scanning directions. This requires an intricate hologram scanner, which limits the increase in the number of scanning directions. On the other hand, if a set number of scanning lines provided by a hologram disk is used, they must be separately assigned according to the plurality of scanning directions required. This means that the number of scanning lines in each direction is decreased, and thus the number of scanning lines is insufficient.